The simplest optical system is a point to point system. This can be unidirectional or bidirectional. These were the first optical systems to be deployed even before the advent of optical amplification. Single wavelengths modulated with the information being transmitted would be coupled into and out of an optical fiber. The optical signal was then converted to/from electrical on either end of the fiber span, a practice called regeneration. The length of the optical span was limited mainly by the loss of the fibre. The advent of optical amplifiers allowed the transmission of the optical signal through multiple fibre spans by overcoming the loss of the fibre without electrical regeneration. They also allowed amplification of more than one optical channel within the same fibre, which allowed the introduction of wavelength division multiplexing (WDM). The transmission distance (length of span multiplied by the number of spans) was now limited by the chromatic dispersion of the fibre and the noise of the optical amplifiers. Optical dispersion compensators were introduced to combat dispersion, which extended the distance of propagation to the point that the limiting factors became the noise limits of the amplifiers and the non-linear interactions of the optical channels with the fibre itself. The advent of optical add/drop multiplexing (OADM) allowed these systems to serve intermediate locations other then the end points of the system.
A major disadvantage of the point to point system is system availability. If there is a failure of a piece of equipment or a cut of a fibre span, the system is immediately unavailable. Therefore these systems have been deployed in pairs, a working and a protection system which duplicated the equipment and fibre requirements. These systems are also bounded by electrical interconnections, which require electrical to optical and optical to electrical conversions.
Ring-based systems have been extensively deployed as a means to overcome the system availability limitations of point to point links by providing resiliency to fibre cuts and equipment failure within a single system, for example the bi-directional line-switched ring (BLSR). Ring based deployments are the norm for most high-availability deployments, however, the ring architecture does not always lend itself to the natural geographical layout of nodes in a service area. Extensive work has been done to build rings out of arbitrary demands and locations, or to provide sets of smaller rings which are connected by larger rings. These interconnections are done using electrical cross-connects, for example a Synchronous Optical Network Add/Drop Multiplexer (SONET ADM). The line-switching function of such a system relies on the detection and conversion of the optical signals into electrical signals and therefore the interconnection of such ring-based systems has remained electrical.
Another problem that needs to be addressed is the growth of optical networks, for example to satisfy increasing demand, or to expand the geographic reach of the network, or to interconnect existing networks.
The creation of a communications network which contains multiple optical systems, either ring-based or point-to-point or a combination of both, created the need for a means to control and analyze the overall network. A typical approach introduces a hierarchy of systems wherein there is a top-level Network Management System (NMS) which was responsible for all subtending systems.
Mesh architectures for optical networks have been developed in order to address complexity, scalability and flexibility issues with conventional ring or point-to-point networks. It would be best if the mesh itself could be relatively arbitrary in its topology in order to best fit the varying geographical and traffic demands, and to accommodate growth in said networks, with the least cost and effort. In order to support and interconnect various networks, the chosen architecture should support both mesh and ring deployments including ring-to-ring interconnect systems.
Current implementations of control planes, such as GMPLS, have grown from electrical switching/routing applications which have no channel interaction implications for performing switches. However this cannot be easily extended to scalable, extendible optical networks, such as optical transmission systems, including the optical amplifiers and the transmission fiber, act in such a way that actions taken on any one or set of channels is not independent of the other channels which are traversing the system. For example power transients in one set of channels cause power fluctuations on other channels through interactions such as amplifier spectral gain ripple and Stimulated Raman Scattering (SRS). Furthermore these can be increased by additional fluctuations caused by cascading control systems through which the signals pass. Note that these cascaded non-linear effects which are introduced by nodes in the system should be distinguished from the distortion of channels which occurs through non-linear interactions as the signal propagates through a waveguide, such as Cross Phase Modulation (XPM) and Four Wave Mixing (FWM).
One method of addressing this problem is to use optical-to-electrical conversion and then subsequent electrical-to-optical conversion. However, Electrical interconnection of optical systems requires that all of the optical channels are demultiplexed and treated individually with electrical regenerators which is costly, requires high power consumption, and requires a great deal of space.
Accordingly there is a need to improve the control and deployment of complex optical networks.